I'm curious how geomason handles geographic references. I'm importing a shapefile with polygons in the WGS84 reference system. All the points have values like 52.03N 7.63E. When I measure the polygon area with a gis then I get different values than using the getArea on the polygons geometry.

Even when I accept that getArea gives me the area in degrees it isn't the same m² after I convert the value.

I found nothing in the api or the other doc sources where this part is described.

Now I'm not sure what to do. Am I making a mistake or do I miss something?

GeoMASON is not a GIS; its geometric operations are geospatial agnostic in that it treats geometry as plain geometry. So, in the case of WGS84 data that's really on an ellipsoid, you'll get different values from GeoMASON because the areas it calculates are on the Euclidean plane and not an ellipsoid.

If it's important for you to have access to the actual area from within GeoMASON, I suggest calculating the area within a proper GIS application, adding that as an attribute, and then accessing that attribute from within the simulation. An alternative approach is to reproject the data to UTM, which is in meters and "flattened", and then calculate the area from within GeoMASON. However, be aware that projecting warps the underlying data, so it still won't be 100% accurate.